Phase noise of signal sources is a severe cause of performance degradation in communication systems. Thus, in the production phase of radio frequency (RF) chips that are configured to handle electrical, optical and/or electromagnetic signals in the radio frequency range (3 KHz up to 300 THz), it is very important to accurately measure the phase noise of the device under test (DUT), meanwhile keeping the cost of test (COT) as low as possible.
Delay line discriminator techniques are well known for phase noise measurement and offer the advantage of avoiding external sources with good phase noise properties, for example a local oscillator having a substantially lower phase noise than the device under test. Especially for measurement of phase noise of highest performance devices under test a local oscillator is expected to exhibit less than state of the art phase noise. Delay line discriminator techniques are a phase noise measurement method which does not require a local oscillator. The radio frequency signal to be measured is split in two paths: one path is passed through a tunable phase shifter, and the second path is passed through a delay line. The signal at the output of the phase shifter is then mixed with the signal at the output of the delay line. Delay line discriminators provide a relatively easy and low cost solution for phase noise measurement.
A description of a conventional delay line discriminator can be found in the Microwave Journal of December 1983, page 103 ff.: “Theory and Design of the Delay Line Discriminator for Phase Noise Measurements” by Christopher Schiebold.
However, delay line discriminators are known to suffer severe sensitivity loss for close-in phase noise measurements showing a poor performance, due to the non-zero correlation between the phase noise waveform and its delayed version. The poor performance for close-in phase measurements is due to the strong attenuation that low frequency components undergo when passing through the delay line discriminator. This attenuation can be so strong that the signal falls below the noise floor, making the phase noise measurement virtually impossible. The correlation between the phase noise waveform and its delayed version can be decreased and thus the sensitivity improved by using longer delay lines. However, long analog delay lines are difficult to build.